### Chapter 6: Arrays

```Chapter 6: Arrays
Presentation slides for
Java Software Solutions
for AP* Computer Science
by John Lewis, William Loftus, and Cara Cocking
Presentation slides are copyright 2002 by John Lewis, William Loftus, and Cara Cocking. All rights
reserved.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
*AP is a registered trademark of The College Entrance Examination Board which was not involved in
the production of, and does not endorse, this product.
Arrays
 Arrays are objects that help us organize large
amounts of information
 Chapter 6 focuses on:
•
•
•
•
•
•
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array declaration and use
passing arrays and array elements as parameters
arrays of objects
searching an array
sorting elements in an array
hashing
two-dimensional arrays
the ArrayList class
• polygons, polylines, and more button components
2
Arrays
 An array is an ordered list of values
Each value has a numeric index
The entire array
has a single name
0
scores
1
2
3
4
5
6
7
8
9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
This array holds 10 values that are indexed from 0 to 9
3
Arrays
 A particular value in an array is referenced using the
array name followed by the index in brackets
 For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
 That expression represents a place to store a single
integer and can be used wherever an integer variable
can be used
4
Arrays
 For example, an array element can be assigned a
value, printed, or used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
System.out.println ("Top = " + scores[5]);
5
Arrays
 The values held in an array are called array elements
 An array stores multiple values of the same type (the
element type)
 The element type can be a primitive type or an object
reference
 Therefore, we can create an array of integers, or an
array of characters, or an array of String objects,
etc.
 In Java, the array itself is an object
 Therefore the name of the array is a object reference
variable, and the array itself must be instantiated
6
Declaring Arrays
 The scores array could be declared as follows:
int[] scores = new int[10];
 The type of the variable scores is int[] (an array of
integers)
 Note that the type of the array does not specify its
size, but each object of that type has a specific size
 The reference variable scores is set to a new array
object that can hold 10 integers
 See BasicArray.java (page 300)
7
Declaring Arrays
 Some examples of array declarations:
double[] prices = new double[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
8
Bounds Checking
 Once an array is created, it has a fixed size
 An index used in an array reference must specify a
valid element
 That is, the index value must be in bounds (0 to N-1)
 The Java interpreter throws an
ArrayIndexOutOfBoundsException if an array
index is out of bounds
 This is called automatic bounds checking
9
Bounds Checking
 For example, if the array codes can hold 100 values,
it can be indexed using only the numbers 0 to 99
 If count has the value 100, then the following
reference will cause an exception to be thrown:
System.out.println (codes[count]);
 It’s common to introduce off-by-one errors when
using arrays
problem
for (int index=0; index <= 100; index++)
codes[index] = index*50 + epsilon;
10
Bounds Checking
 Each array object has a public constant called
length that stores the size of the array
 It is referenced using the array name:
scores.length
 Note that length holds the number of elements, not
the largest index
 See ReverseOrder.java (page 302)
 See LetterCount.java (page 304)
11
Initializer Lists
 An initializer list can be used to instantiate and
initialize an array in one step
 The values are delimited by braces and separated by
commas
 Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};
12
Initializer Lists
 Note that when an initializer list is used:
• the new operator is not used
• no size value is specified
 The size of the array is determined by the number of
items in the initializer list
 An initializer list can only be used only in the array
declaration
 See Primes.java (page 308)
13
Arrays as Parameters
 An entire array can be passed as a parameter to a
method
 Like any other object, the reference to the array is
passed, making the formal and actual parameters
aliases of each other
 Changing an array element within the method
changes the original
 An array element can be passed to a method as well,
and follows the parameter passing rules of that
element's type
14
Arrays of Objects
 The elements of an array can be object references
 The following declaration reserves space to store 25
references to String objects
String[] words = new String[25];
 It does NOT create the String objects themselves
 Each object stored in an array must be instantiated
separately
15
Command-Line Arguments
 The signature of the main method indicates that it
takes an array of String objects as a parameter
 These values come from command-line arguments
that are provided when the interpreter is invoked
 For example, the following invocation of the
interpreter passes an array of three String objects
into main:
> java StateEval pennsylvania texas arizona
 These strings are stored at indexes 0-2 of the
parameter
 See NameTag.java (page 311)
16
Arrays of Objects
 Objects can have arrays as instance variables
 Many useful structures can be created with arrays
and objects
 The software designer must determine carefully an
organization of data and objects that makes sense
for the situation
 See Tunes.java (page 312)
 See CDCollection.java (page 314)
 See CD.java (page 316)
17
Searching
 A common task when working with arrays is to
search an array for a particular element
 A linear or sequential search examines each element
of the array in turn until the desired element is found
 A binary search is more efficient than a linear search
but it can only be performed on an ordered list
 A binary search examines the middle element and
moves left if the desired element is less than the
middle, and right if the desired element is greater
 This process repeats until the desired element is
found
 See Searches.java (page 319)
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Sorting
 Sorting is the process of arranging a list of items in a
particular order
 The sorting process is based on specific value(s)
• sorting a list of test scores in ascending numeric order
• sorting a list of people alphabetically by last name
 There are many algorithms for sorting a list of items
 These algorithms vary in efficiency
 We will examine two specific algorithms:
• Selection Sort
• Insertion Sort
19
Selection Sort
 The approach of Selection Sort:
• select a value and put it in its final place into the list
• repeat for all other values
 In more detail:
• find the smallest value in the list
• switch it with the value in the first position
• find the next smallest value in the list
• switch it with the value in the second position
• repeat until all values are in their proper places
20
Selection Sort
 An example:
original:
smallest is
smallest is
smallest is
smallest is
1:
2:
3:
6:
3
1
1
1
1
9
9
2
2
2
6
6
6
3
3
1
3
3
6
6
2
2
9
9
9
 See Sorts.java (page 324) -- the selectionSort
method
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Swapping
 Swapping is the process of exchanging two values
 Swapping requires three assignment statements
temp = first;
first = second;
second = temp;
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Insertion Sort
 The approach of Insertion Sort:
• pick any item and insert it into its proper place in a sorted
sublist
• repeat until all items have been inserted
 In more detail:
• consider the first item to be a sorted sublist (of one item)
• insert the second item into the sorted sublist, shifting the
first item as needed to make room to insert the new addition
• insert the third item into the sorted sublist (of two items),
shifting items as necessary
• repeat until all values are inserted into their proper positions
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Insertion Sort
 An example:
original:
insert 9:
insert 6:
insert 1:
insert 2:
3
3
3
1
1
9
9
6
3
2
6
6
9
6
3
1
1
1
9
6
2
2
2
2
9
 See Sorts.java (page 324) -- the insertionSort
method
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Sorting Objects
 Integers have an inherent order, but the ordering
criteria of a collection of objects must be defined
 Recall that a Java interface can be used as a type
name and guarantees that a particular class
implements particular methods
 We can use the Comparable interface and the
compareTo method to develop a generic sort for a set
of objects
 See SortPhoneList.java (page 328)
 See Contact.java (page 329)
 See Sorts.java (page 324) – the second
insertionSort method
25
Comparing Sorts
 Time efficiency refers to how long it takes an
algorithm to run
 Space efficiency refers to the amount of space an
algorithm uses
 Algorithms are compared to each other by
expressing their efficiency in big-oh notation
 An efficiency of O(n) is better than O(n2), where n
refers to the size of the input
 Time efficiency O(2n) means that as the size of the
input increases, the running time increases
exponentially
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Comparing Sorts
 Both Selection and Insertion sorts are similar in
efficiency
 They both have outer loops that scan all elements,
and inner loops that compare the value of the outer
loop with almost all values in the list
 Approximately n2 number of comparisons are made
to sort a list of size n
 We therefore say that these sorts have efficiency
O(n2), or are of order n2
 Other sorts are more efficient: O(n log2 n)
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Hashing
 Hashing is a technique used to efficiently store and
retrieve data in an array
 An array used for hashing is called a hash table
 A hash function calculates a hash code for each data
item.
 The hash code is used as an index into the array,
telling where the data item should be stored
 Example: hash function f(n) = n % 7
• Element 18 would be stored in array cell 18 % 7 or 4
28
Two-Dimensional Arrays
 A one-dimensional array stores a list of elements
 A two-dimensional array can be thought of as a table
of elements, with rows and columns
one
dimension
two
dimensions
29
Two-Dimensional Arrays
 To be precise, a two-dimensional array in Java is an
array of arrays
 A two-dimensional array is declared by specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
 A two-dimensional array element is referenced using
two index values
value = scores[3][6]
 The array stored in one row or column can be
specified using one index
30
Two-Dimensional Arrays
Expression
scores
Type
int[][]
Description
scores[5]
int[]
array of integers
scores[5][12]
int
integer
2D array of integers, or
array of integer arrays
 See TwoDArray.java (page 335)
 See SodaSurvey.java (page 336)
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The ArrayList Class
 The ArrayList class is part of the java.util package
 Like an array, it can store a list of values and reference
them with an index
 Unlike an array, an ArrayList object grows and shrinks
as needed
 Items can be inserted or removed with a single method
invocation
 It stores references to the Object class, which allows it to
store any kind of object
 See DestinysChild.java (page 339)
 See Recipe.java (page 341)
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ArrayList Efficiency
 The ArrayList class is implemented using an array
 The code of the ArrayList class automatically
expands the array's capacity to accommodate
 The array is manipulated so that indexes remain
continuous as elements are added or removed
 If elements are added to and removed from the end of
the list, this processing is fairly efficient
 If elements are inserted and removed from the middle
of the list, the elements are constantly being shifted
around
33
Polygons and Polylines
 Arrays often are helpful in graphics processing
 Polygons and polylines are shapes that can be
defined by values stored in arrays
 A polyline is similar to a polygon except that its
endpoints do not meet, and it cannot be filled
 See Rocket.java (page 359)
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The Rocket Program
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The Polygon Class
 The Polygon class, defined in the java.awt package
can be used to define and draw a polygon
 Two versions of the overloaded drawPolygon and
fillPolygon methods each take a single Polygon
object as a parameter
 A Polygon object encapsulates the coordinates of
the polygon
36
Check Boxes
 A check box is a button that can be toggled on or off
 A check box is represented by the JCheckBox class
 A change of state generates an item event
 The ItemListener interface corresponds to item
events
 The itemStateChanged method of the listener
responds when a check box changes state
37
The StyleOptions Program
 A frame is a container that can be used to create
stand-alone GUI applications
 A frame is represented by the JFrame class
 A Font object represents by the font's:
• family name (such as Times or Courier)
• style (bold, italic, or both)
• font size
 See StyleOptions.java (page 363)
 See StyleGUI.java (page 364)
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The StyleOptions Program
39
 A set of radio buttons represents a set of mutually
exclusive options
 When a radio button from a group is selected, the
other button currently "on" in the group is toggled off
 A radio button generates an action event
 See QuoteOptions.java (page 366)
 See QuoteGUI.java (page 368)
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The QuoteOptions Program
41
Summary
 Chapter 6 has focused on:
•
•
•
•
•
•
•
•
array declaration and use
passing arrays and array elements as parameters
arrays of objects
searching an array
sorting elements in an array
hashing
two-dimensional arrays
the ArrayList class
• polygons, polylines, and more button components
42
```